In devices for manufacturing semiconductor components, it is necessary to precisely determine the spatial position of certain parts which are movable relative to each other, using suitable position-measuring devices. A computer-controlled sequencing control is possible in these devices based on the position information ascertained. In certain conventional methods, the position measuring necessary to this end has been performed predominantly by a plurality of laser interferometers. In the future, it must be assumed that the accuracy requirements of the position measuring, accompanied at the same time by increasing traversing speeds of the various parts, will further increase. Given the stringent accuracy requirements then resulting, laser interferometers can no longer be used as position-measuring devices. The refractive-index fluctuations in the ambient air, even in the case of optimal air conditioning, lead to unacceptable measured-value fluctuations on the order of several nanometers (nm) in the position determination.
For this reason, alternative position-measuring devices have already been proposed for devices of this type. For example, European Published Patent Application No. 1 019 669 describes the use of optical position-measuring devices having what are termed grid gratings as a two-dimensional measuring standard. Such position-measuring devices shall also be denoted hereinafter as grating-based position-measuring devices. These systems are scarcely influenced by possible fluctuations in the refractive index of the air, and therefore allow easily reproducible position measurements.
Optical position-measuring devices having gratings as measuring standards which supply the necessary resolutions in the nanometer range are usually based on interferential scanning principles. In this connection, as a rule, a light beam from a suitable light source is split into at least two coherent partial beams of rays which subsequently act upon a plurality of gratings in the respective partial-beam paths before they are reunited and brought to interference. The position information ultimately of interest is provided by the (displacement-dependent) phase position of the two interfering partial beams of rays. The resulting path-length difference is usually approximately zero for the two partial beams of rays between splitting and reuniting in symmetrical partial-beam path variants of such systems. Therefore, a small coherence length of the light used is sufficient to ensure the desired interference on the detection side.
German Published Patent Application No. 10 2005 043 569 describes a further interferential position-measuring device which preferably has asymmetrical partial-beam paths for the two partial beams of rays. That is, because of this asymmetry, path-length differences on the order of up to several millimeters (mm) result for the partial beams of rays arriving at interference. With regard to the necessary coherence length of the light used, this means that the coherence length must lie in the range of several millimeters (mm) up to several centimeters (cm); otherwise, no interference of the partial beams of rays coming to superimposition is possible. A light source which basically satisfies the demands discussed above in an extremely precise position-measuring device is described in German Published Patent Application No. 10 2006 041 357. It takes the form of a semiconductor laser having a great coherence length in the range between 1 mm and 1 cm, which functions in pulsed fashion in a single-mode operation. For instance, what are termed DFB (distributed feedback) semiconductor lasers or else DBR (distributed Bragg reflector] semiconductor lasers are considered as suitable semiconductor lasers. In general, the described light sources satisfy the demands with respect to the coherence length, however, require a certain degree of technical complexity, especially if high optical powers are needed.
Incidentally, in principle, similar demands on the light source also result if the position-measuring device is in the form of an interferometer which is operated in pulsed fashion.